In a cellular telecommunication system as illustrated in FIG. 1, like a UMTS or LTE system, a plurality of secondary stations 110a-d like User Equipments, communicate within a cell 100a with the primary station 101a operating the cell. In such a system, the primary station 101a and the secondary stations may each comprise an antenna array comprising a plurality of antennas. These antennas may be used to communicate in a MIMO mode by beamforming. Complex coefficients applied on the transmitting antennas of the transmitting station, here the primary station 101a and/or on the receiving station, here the secondary stations 110a-d enable the creation of communication streams each of which are associated with one or more spatial channels.
A spatial channel is defined by the combination of transmission parameters like a modulation sequence, a time/frequency resource, and/or the beamformed stream. Thus, this permits to reach high data rates, and increased communication range.
To achieve such beamforming communication, the secondary stations and the primary stations typically need to be synchronised (i.e. operate with a common time frame) and to have a common phase reference. Reference symbols may be used to facilitate the timing synchronization and to achieve demodulation of the communication stream in a beamforming communication mode. A reference symbol has a predetermined transmitted value which permits the receiving station to have for instance substantially the same phase reference as the transmitting station, or to estimate channel conditions, so that a suitable modulation and coding scheme may be selected at the transmitting station.
In the case of a secondary station receiving a plurality of spatial channel, it is recommended to have at least one reference symbol (preferably a set or sequence of several reference symbols) corresponding to each spatial channel. However, and for instance in the case of the secondary station 110d which is at the edge of the cell 100a, the reference symbols transmitted from a primary station 101b of a neighboring cell 100b may collide with the reference symbols associated with spatial channels of cell 100a. There is thus a need to avoid or mitigate the effects of such collisions.
This problem of collision may also happen between reference symbols of a single cell, for example, where reference symbols are transmitted to more than one secondary station.